Most troublingly, it appears that his cerebellum is shrinking and after years of tests his doctors still don’t know why, according to his mother Natasha Durant.

Cerebellum shrinking "can cause a lot of problems. It may be responsible for nerve degeneration. It may also cause problems with motor function and speech,” Durant told ABCNews.com.

His doctors told Durant that they believe a genetic abnormality is most likely the cause of Corbin’s symptoms, however, multiple tests looking for different genetic disorders have come back negative and a diagnosis remains elusive.

In recent months Corbin’s doctors have recommended that the energetic boy be taken to the Mayo Clinic in the hopes that they would be able to figure out his diagnosis.

His doctors "were working on this for four years. They decided it’s time for him to see a new team and almost all of them first mentioned Mayo Clinic,” said Durant. “If we can figure out a diagnosis … I’m hoping there’s something to slow it down.”

While the disease has taken a physical toll, Durant said Corbin remains a very outgoing kid and he appears to be aware of how other people perceive his illness.

“People always ask what’s wrong,” Durant said of strangers when they see Corbin for the first time. “Up until three months ago he didn’t have a care in the world. ... [But] he’s starting to understand.”